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Encyclopedia > Igneous rock
World geologic provinces (USGS) Oceanic crust      0-20 Ma      20-65 Ma      >65 Ma Geologic province      Shield      Platform      Orogen      Basin      Large igneous province      Extended crust
World geologic provinces (USGS)
Oceanic crust      0-20 Ma      20-65 Ma      >65 Ma Geologic province      Shield      Platform      Orogen      Basin      Large igneous province      Extended crust

Igneous rocks (etymology from Latin ignis, fire) are rocks formed by solidification of cooled magma (molten rock), with or without crystallization, either below the surface as intrusive (plutonic) rocks or on the surface as extrusive (volcanic) rocks. This magma can be derived from partial melts of pre-existing rocks in either the Earth's mantle or crust. Typically, the melting is caused by one or more of the following processes — an increase in temperature, a decrease in pressure, or a change in composition. Over 700 types of igneous rocks have been described, most of them formed beneath the surface of the Earth's crust. Rock redirects here. ... Magma is molten rock located beneath the surface of the Earth (or any other terrestrial planet), and which often collects in a magma chamber. ... Frost crystallization on a shrub. ... Devils Tower, an igneous intrusion exposed when the surrounding softer rock eroded away. ... In geology an intrusion is usually a body of igneous rock that has crystallized from a molten magma below the surface of the Earth. ... Extrusive refers to a mode of igneous rock formation, in which hot magma from inside the Earth flows out (extrudes) onto the surface. ... Ignimbrite is a deposit of a pyroclastic flow. ... This article is about Earth as a planet. ... Earth cutaway from core to exosphere. ... World geologic provinces (USGS) Oceanic crust  0-20 Ma  20-65 Ma  >65 Ma Geologic province  Shield  Platform  Orogen  Basin  Large igneous province  Extended crust In geology, a crust is the outermost solid shell of a planet or moon. ... World geologic provinces (USGS) Oceanic crust  0-20 Ma  20-65 Ma  >65 Ma Geologic province  Shield  Platform  Orogen  Basin  Large igneous province  Extended crust In geology, a crust is the outermost solid shell of a planet or moon. ...

Contents

Geologic significance

Igneous rocks make up approximately ninety-five percent of the upper part of the Earth's crust, but their great abundance is hidden on the Earth's surface by a relatively thin but widespread layer of sedimentary and metamorphic rocks. Two types of sedimentary rock: limey shale overlaid by limestone. ... Quartzite, a form of metamorphic rock, from the Museum of Geology at University of Tartu collection. ...


Igneous rocks are geologically important because:

  • their minerals and global chemistry give information about the composition of the mantle, from which some igneous rocks are extracted, and the temperature and pressure conditions that allowed this extraction, and/or of other pre-existing rock that melted;
  • their absolute ages can be obtained from various forms of radiometric dating and thus can be compared to adjacent geological strata, allowing a time sequence of events;
  • their features are usually characteristic of a specific tectonic environment, allowing tectonic reconstitutions (see plate tectonics);
  • in some special circumstances they host important mineral deposits (ores): for example, tungsten, tin, and uranium are commonly associated with granites, whereas ores of chromium and platinum are commonly associated with gabbros.

Radiometric dating (often called radioactive dating) is a technique used to date materials, based on a comparison between the observed abundance of particular naturally occurring radioactive isotopes and their known decay rates. ... For other uses, see strata (novel) and strata title. ... The tectonic plates of the world were mapped in the second half of the 20th century. ... For other uses, see Ore (disambiguation). ... For other uses, see Tungsten (disambiguation). ... This article is about the metallic chemical element. ... This article is about the chemical element. ... For other uses, see granite (disambiguation). ... REDIRECT [[ Insert text]]EWWWWWWWWWWWWW YO General Name, symbol, number chromium, Cr, 24 Chemical series transition metals Group, period, block 6, 4, d Appearance silvery metallic Standard atomic weight 51. ... General Name, Symbol, Number platinum, Pt, 78 Chemical series transition metals Group, Period, Block 10, 6, d Appearance grayish white Standard atomic weight 195. ... Gabbro specimen. ...

Morphology and setting

In terms of modes of occurrence, igneous rocks can be either intrusive (plutonic) or extrusive (volcanic). Devils Tower, an igneous intrusion exposed when the surrounding softer rock eroded away. ... Extrusive refers to a mode of igneous rock formation, in which hot magma from inside the Earth flows out (extrudes) onto the surface. ... Ignimbrite is a deposit of a pyroclastic flow. ...


Intrusive igneous rocks

Intrusive igneous rocks are formed from magma that cools and solidifies within the earth. Surrounded by pre-existing rock (called country rock), the magma cools slowly, and as a result these rocks are coarse grained. The mineral grains in such rocks can generally be identified with the naked eye. Intrusive rocks can also be classified according to the shape and size of the intrusive body and its relation to the other formations into which it intrudes. Typical intrusive formations are batholiths, stocks, laccoliths, sills and dikes. The extrusive rocks often produce lava flows. Pluton redirects here. ... Half Dome, a granite monolith in Yosemite National Park and part of the Sierra Nevada batholith. ... A laccolith is an igneous intrusion (or concordant pluton) that has been injected between two layers of sedimentary rock. ... In geology, a sill is a tabular, often horizontal mass of igneous rock that has been intruded laterally between older layers of sedimentary rock, beds of volcanic lava or tuff, or even along the direction of foliation in metamorphic rock. ... A dike in geology refers to a tabular intrusive igneous body. ... Look up lava, Aa, pahoehoe in Wiktionary, the free dictionary. ...


The central cores of major mountain ranges consist of intrusive igneous rocks, usually granite. When exposed by erosion, these cores (called batholiths) may occupy huge areas of the Earth's surface. For other uses, see granite (disambiguation). ... Half Dome, a granite monolith in Yosemite National Park and part of the Sierra Nevada batholith. ...


Coarse grained intrusive igneous rocks which form at depth within the earth are termed as abyssal; intrusive igneous rocks which form near the surface are termed hypabyssal.

Igneous rock: light colored tracks show the direction of lava flow
Igneous rock: light colored tracks show the direction of lava flow

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Extrusive igneous rocks

Extrusive igneous rocks are formed at the Earth's surface as a result of the partial melting of rocks within the mantle and crust. Earth cutaway from core to exosphere. ...


The melt, with or without suspended crystals and gas bubbles, is called magma. Magma rises because it is less dense than the rock from which it was created. When it reaches the surface, magma extruded onto the surface either beneath water or air, is called lava. Eruptions of volcanoes into air are termed subaerial whereas those occurring underneath the ocean are termed submarine. Black smokers and mid-ocean ridge basalt are examples of submarine volcanic activity. Magma is molten rock located beneath the surface of the Earth (or any other terrestrial planet), and which often collects in a magma chamber. ... Look up lava, Aa, pahoehoe in Wiktionary, the free dictionary. ... This article is about volcanoes in geology. ... The term subaerial, mainly used in geology, describes events or structures located at the Earths surface, under the air. This is to be contrasted with submarine events or structures, those located under the sea. ... A black smoker in the Atlantic Ocean Black smokers are a type of hydrothermal vent found on the ocean floor. ... Oceanic Ridge Oceanic crust is formed at an oceanic ridge, while the lithosphere is subducted back into the asthenosphere at trenches. ... For the cities, see Basalt, Colorado and Basalt, Idaho. ...


The volume of extrusive rock erupted annually by volcanoes varies with plate tectonic setting. Extrusive rock is produced in the following proportions:[1]

Magma which erupts from a volcano behaves according to its viscosity, determined by temperature, composition, and crystal content. High-temperature magma, most of which is basaltic in composition, behaves in a manner similar to thick oil and, as it cools, treacle. Long, thin basalt flows with pahoehoe surfaces are common. Intermediate composition magma such as andesite tends to form cinder cones of intermingled ash, tuff and lava, and may have viscosity similar to thick, cold molasses or even rubber when erupted. Felsic magma such as rhyolite is usually erupted at low temperature and is up to 10,000 times as viscous as basalt. Volcanoes with rhyolitic magma commonly erupt explosively, and rhyolitic lava flows typically are of limited extent and have steep margins, because the magma is so viscous. In plate tectonics, a divergent boundary (divergent fault boundary or divergent plate boundary), (but also known as a constructive boundary or an extensional boundary) is a linear feature that exists between two tectonic plates that are moving away from each other. ... In plate tectonics, a convergent boundary – also known as a convergent plate boundary or a destructive plate boundary – is an actively deforming region where two (or more) tectonic plates or fragments of lithosphere move toward one another. ... Categories: Geology stubs | Plate tectonics ... In geology, a hotspot is a location on the Earths surface that has experienced active volcanism for a long period of time. ... Cleveland Volcano in the Aleutian Islands of Alaska photographed from the International Space Station For other uses, see Volcano (disambiguation). ... For other uses, see Viscosity (disambiguation). ... Lava is molten rock that a volcano expels during an eruption. ... A sample of andesite (dark groundmass) with amygdaloidal vesicules filled with zeolite. ... Ash plume from Mt Cleveland, a stratovolcano Diamond Head, a well-known backdrop to Waikiki in Hawaii, is an ash cone that solidified into tuff Volcanic ash consists of very fine rock and mineral particles less than 2 mm in diameter that are ejected from a volcanic vent. ... Welded tuff at Golden Gate in Yellowstone National Park Tuff (from the Italian tufo) is a type of rock consisting of consolidated volcanic ash ejected from vents during a volcanic eruption. ... Molasses or treacle is a thick syrup by-product from the processing of the sugarcane or sugar beet into sugar. ... This page is about a volcanic rock. ... For the cities, see Basalt, Colorado and Basalt, Idaho. ...


Felsic and intermediate magmas that erupt often do so violently, with explosions driven by release of dissolved gases — typically water but also carbon dioxide. Explosively erupted pyroclastic material is called tephra and includes tuff, agglomerate and ignimbrite. Fine volcanic ash is also erupted and forms ash tuff deposits which can often cover vast areas. Carbon dioxide is a chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom. ... Pyroclastic rocks are formed from lavas which are ejected into the air, as occur in pyroclastic flows or Plinian eruptions. ... Tephra refers to air-fall material produced by a volcanic eruption regardless of composition or fragment size. ... Welded tuff at Golden Gate in Yellowstone National Park Tuff (from the Italian tufo) is a type of rock consisting of consolidated volcanic ash ejected from vents during a volcanic eruption. ... Agglomerate - Wikipedia /**/ @import /skins-1. ... Ignimbrite is a volcanic pyroclastic rock, often of dacitic or rhyolitic composition. ... Ash plume from Mt Cleveland, a stratovolcano Diamond Head, a well-known backdrop to Waikiki in Hawaii, is an ash cone that solidified into tuff Volcanic ash consists of very fine rock and mineral particles less than 2 mm in diameter that are ejected from a volcanic vent. ...


Because lava cools and crystallizes rapidly, it is fine grained. If the cooling has been so rapid as to prevent the formation of even small crystals after extrusion, the resulting rock may be mostly glass (such as the rock obsidian). If the cooling of the lava happened slowly, the rocks would be coarse-grained. This article is about a type of volcanic glass. ...


Because the minerals are mostly fine-grained, it is much more difficult to distinguish between the different types of extrusive igneous rocks than between different types of intrusive igneous rocks. Generally, the mineral constituents of fine-grained extrusive igneous rocks can only be determined by examination of thin sections of the rock under a microscope, so only an approximate classification can usually be made in the field. Photomicrograph of a thin section of gabbro. ... A 1879 Carl Zeiss Jena Optical microscope. ...


Classification

Igneous rocks are classified according to mode of occurrence, texture, mineralogy, chemical composition, and the geometry of the igneous body.


The classification of the many types of different igneous rocks can provide us with important information about the conditions under which they formed. Two important variables used for the classification of igneous rocks are particle size, which largely depends upon the cooling history, and the mineral composition of the rock. Feldspars, quartz or feldspathoids, olivines, pyroxenes, amphiboles, and micas are all important minerals in the formation of almost all igneous rocks, and they are basic to the classification of these rocks. All other minerals present are regarded as nonessential in almost all igneous rocks and are called accessory minerals. Types of igneous rocks with other essential minerals are very rare, and these rare rocks include those with essential carbonates. This article does not cite any references or sources. ... For other uses, see Quartz (disambiguation). ... The feldspathoids are a group of tectosilicate minerals which resemble feldspars but have a different structure and much lower silica content. ... The mineral olivine (also called chrysolite and, when gem-quality, peridot) is a magnesium iron silicate with the formula (Mg,Fe)2SiO4. ... Figure 1:Mantle-peridotite xenolith with green peridot olivine and black pyroxene crystals from San Carlos Indian Reservation, Gila Co. ... For the logical fallacy, see Amphibology. ... Rock with mica Mica sheet Mica flakes The mica group of sheet silicate minerals includes several closely related materials having highly perfect basal cleavage. ... Ball-and-stick model of the carbonate ion, CO32− For other meanings, see Carbonate (disambiguation) In chemistry, a carbonate is a salt or ester of carbonic acid. ...


In a simplified classification, igneous rock types are separated on the basis of the type of feldspar present, the presence or absence of quartz, and in rocks with no feldspar or quartz, the type of iron or magnesium minerals present. Rocks containing quartz (silica in composition) are silica-oversaturated. Rocks with feldspathoids are silica-undersaturated, because feldspathoids cannot coexist in a stable association with quartz. For other uses, see Quartz (disambiguation). ... The feldspathoids are a group of tectosilicate minerals which resemble feldspars but have a different structure and much lower silica content. ...


Igneous rocks which have crystals large enough to be seen by the naked eye are called phaneritic; those with crystals too small to be seen are called aphanitic. Generally speaking, phaneritic implies an intrusive origin; aphanitic an extrusive one.


An igneous rock with larger, clearly discernible crystals embedded in a finer-grained matrix is termed porphyry. Porphyritic texture develops when some of the crystals grow to considerable size before the main mass of the magma crystallizes as finer-grained, uniform material. A piece of porphyry Porphyry is a variety of igneous rock consisting of large-grained crystals, such as feldspar or quartz, dispersed in a fine-grained feldspathic matrix or groundmass. ...


Texture

Main article: Rock microstructure

Texture is an important criterion for the naming of volcanic rocks. The texture of volcanic rocks, including the size, shape, orientation, and distribution of mineral grains and the intergrain relationships, will determine whether the rock is termed a tuff, a pyroclastic lava or a simple lava. Rock microstructure includes the texture of a rock and the small scale rock structures. ... Rock microstructure includes the texture of a rock and the small scale rock structures. ... For other uses, see Mineral (disambiguation). ... Welded tuff at Golden Gate in Yellowstone National Park Tuff (from the Italian tufo) is a type of rock consisting of consolidated volcanic ash ejected from vents during a volcanic eruption. ... Pyroclastic rocks are formed from lavas which are ejected into the air, as occur in pyroclastic flows or Plinian eruptions. ... Look up lava, Aa, pahoehoe in Wiktionary, the free dictionary. ...


However, the texture is only a subordinate part of classifying volcanic rocks, as most often there needs to be chemical information gleaned from rocks with extremely fine-grained groundmass or from airfall tuffs, which may be formed from volcanic ash. Ash plume from Mt Cleveland, a stratovolcano Diamond Head, a well-known backdrop to Waikiki in Hawaii, is an ash cone that solidified into tuff Volcanic ash consists of very fine rock and mineral particles less than 2 mm in diameter that are ejected from a volcanic vent. ...


Textural criteria are less critical in classifying intrusive rocks where the majority of minerals will be visible to the naked eye or at least using a hand lens, magnifying glass or microscope. Plutonic rocks tend also to be less texturally varied and less prone to gaining structural fabrics. Textural terms can be used to differentiate different intrusive phases of large plutons, for instance porphyritic margins to large intrusive bodies, porphyry stocks and subvolcanic dikes (apophyses). Mineralogical classification is used most often to classify plutonic rocks. Chemical classifications are preferred to classify volcanic rocks, with phenocryst species used as a prefix, e.g. "olivine-bearing picrite" or "orthoclase-phyric rhyolite". (For other meanings of Porphyr, see Porphyry) The baptismal font in the Cathedral of Magdeburg is made of rose porphyry from a site near Assuan, Egypt Porphyry is a very hard red, green or purple igneous rock consisting of large-grained crystals, such as feldspar or quartz, dispersed in a... A piece of porphyry Porphyry is a variety of igneous rock consisting of large-grained crystals, such as feldspar or quartz, dispersed in a fine-grained feldspathic matrix or groundmass. ... A dike in geology refers to a tabular intrusive igneous body. ...

This page is intended to be a list of rock textural and morphological terms. ... Igneous Textures This term is applied to igneous rocks in order to discribe their appearance. ...

Chemical classification

Igneous rocks can be classified according to chemical or mineralogical parameters:


Chemical: total alkali-silica content (TAS diagram) for volcanic rock classification used when modal or mineralogic data is unavailable: The TAS classification can be used to assign names to many common types of volcanic rocks based upon the relationships between the combined alkali content and the silica content. ... Ignimbrite is a deposit of a pyroclastic flow. ...

  • acid igneous rocks containing a high silica content, greater than 63% SiO2 (examples granite and rhyolite)
  • intermediate igneous rocks containing between 52 - 63% SiO2 (example andesite and dacite)
  • basic igneous rocks have low silica 45 - 52% and typically high iron - magnesium content (example gabbro and basalt)
  • ultrabasic igneous rocks with less than 45% silica. (examples picrite and komatiite)
  • alkalic igneous rocks with 5 - 15% alkali (K2O + Na2O) content or with a molar ratio of alkali to silica greater than 1:6. (examples phonolite and trachyte)
Note: the acid-basic terminology is used more broadly in older (generally British) geological literature. In current literature felsic-mafic roughly substitutes for acid-basic.

Chemical classification also extends to differentiating rocks which are chemically similar according to the TAS diagram, for instance;
For other uses, see granite (disambiguation). ... This page is about a volcanic rock. ... A sample of andesite (dark groundmass) with amygdaloidal vesicules filled with zeolite. ... Grey, red, black, altered white/tan, flow-banded pumice dacite Dacite (IPA: ) is an igneous, volcanic rock with a high iron content. ... Gabbro specimen. ... For the cities, see Basalt, Colorado and Basalt, Idaho. ... Igneous rock which crystallizes from silicate minerals at the highest temperatures is referred to as ultramafic rock. ... Picrite is an intrusive and extrusive igneous rock. ... Komatiites are ultramafic mantle-derived volcanic rocks. ... Alkaline redirects here. ... The mole (symbol: mol) is the SI base unit that measures an amount of substance. ... Phonolite is an igneous, volcanic (extrusive) rock, of felsic composition, with aphanitic to porphyritic texture. ... A sample of trachyte Trachyte is an igneous, volcanic rock with an aphanitic to porphyritic texture. ...

  • Ultrapotassic; rocks containing molar K2O/Na2O >3
  • Peralkaline; rocks containing molar (K2O + Na2O)/ Al2O3 >1
  • Peraluminous; rocks containing molar (K2O + Na2O)/ Al2O3 <1

An idealized mineralogy (the normative mineralogy) can be calculated from the chemical composition, and the calculation is useful for rocks too fine-grained or too altered for identification of minerals that crystallized from the melt. For instance, normative quartz classifies a rock as silica-oversaturated; an example is rhyolite. A normative feldspathoid classifies a rock as silica-undersaturated; an example is nephelinite. Ultrapotassic igneous rocks are a class of rare, volumetrically minor generally ultramafic or mafic silica-depleted igneous rocks. ... Normative mineralogy is a geochemical calculation of the whole rock geochemistry of a rock sample which estimates the idealised mineralogy of a rock according to the principles of geochemistry. ... This page is about a volcanic rock. ... The feldspathoids are a group of tectosilicate minerals which resemble feldspars but have a different structure and much lower silica content. ... This page is a candidate to be moved to Wiktionary. ...


History of classification

In 1902 a group of American petrographers proposed that all existing classifications of igneous rocks should be discarded and replaced by a "quantitative" classification based on chemical analysis. They showed how vague and often unscientific was much of the existing terminology and argued that as the chemical composition of an igneous rock was its most fundamental characteristic it should be elevated to prime position.


Geological occurrence, structure, mineralogical constitution, the hitherto accepted criteria for the discrimination of rock species were relegated to the background. The completed rock analysis is first to be interpreted in terms of the rock-forming minerals which might be expected to be formed when the magma crystallizes, e.g., quartz feldspars, olivine, akermannite, feldspathoids, magnetite, corundum and so on, and the rocks are divided into groups strictly according to the relative proportion of these minerals to one another.[2] [3]


Mineralogical classification

For volcanic rocks, mineralogy is important in classifying and naming lavas. The most important criterion is the phenocryst species, followed by the groundmass mineralogy. Often, where the groundmass is aphanitic, chemical classification must be used to properly identify a volcanic rock. Example of phenocrysts in rhomb porphyry from the Oslo rift area in Norway A phenocryst is a relatively large and usually conspicuous crystal formed in the mass of a porphyritic igneous rock. ... An aphanite is an igneous rock with a fine-grained structure. ...


Mineralogic contents - felsic versus mafic

  • felsic rock, highest content of silicon, with predominance of quartz, alkali feldspar and/or feldspathoids: the felsic minerals; these rocks (e.g., granite, rhyolite) are usually light coloured, and have low density.
  • mafic rock, lesser content of silicon relative to felsic rocks, with predominance of mafic minerals pyroxenes, olivines and calcic plagioclase; these rocks (example, basalt, gabbro) are usually dark coloured, and have a higher density than felsic rocks.
  • ultramafic rock, lowest content of silicon, with more than 90% of mafic minerals (e.g., dunite).

For intrusive, plutonic and usually phaneritic igneous rocks where all minerals are visible at least via microscope, the mineralogy is used to classify the rock. This usually occurs on ternary diagrams, where the relative proportions of three minerals are used to classify the rock. Felsic is a term used in geology to refer to silicate minerals, magmas, and rocks which are enriched in the lighter elements such as silica, oxygen, aluminium, sodium, and potassium. ... Not to be confused with Silicone. ... This article does not cite any references or sources. ... The feldspathoids are a group of tectosilicate minerals which resemble feldspars but have a different structure and much lower silica content. ... In geology, mafic minerals and rocks are silicate minerals, magmas, and volcanic and intrusive igneous rocks that have relatively high concentrations of the heavier elements. ... Figure 1:Mantle-peridotite xenolith with green peridot olivine and black pyroxene crystals from San Carlos Indian Reservation, Gila Co. ... The mineral olivine (also called chrysolite and, when gem-quality, peridot) is a magnesium iron silicate with the formula (Mg,Fe)2SiO4. ... Lunar Ferroan Anorthosite #60025 (Plagioclase Feldspar). ... Ultramafic (or ultrabasic) rocks are igneous rocks with very low silica content (less than 45%), generally >18% MgO, high FeO, low potassium and are composed of usually greater than 90% mafic minerals (dark colored, high magnesium and iron content). ... Dunite is an igneous, plutonic rock, of ultramafic composition, with coarse grained or phaneritic texture. ... Granite is typical phaneritic igneous rock. ... A Ternary plot or Ternary Graph is a specialization of a Barycentric plot for three variables. ...


The following table is a simple subdivision of igneous rocks according both to their composition and mode of occurrence.

Composition
Mode of occurrence Felsic Intermediate Mafic Ultramafic
Intrusive Granite Diorite Gabbro Peridotite
Extrusive Rhyolite Andesite Basalt Komatiite
Essential rock forming silicates
Felsic Intermediate Mafic Ultramafic
Coarse Grained Granite Diorite Gabbro Peridotite
Medium Grained Diabase
Fine Grained Rhyolite Andesite Basalt Komatiite

For a more detailed classification see QAPF diagram. For other uses, see granite (disambiguation). ... Categories: Mineral stubs | Igneous rocks ... Gabbro specimen. ... Peridotite xenolith from San Carlos, southwestern United States. ... This page is about a volcanic rock. ... A sample of andesite (dark groundmass) with amygdaloidal vesicules filled with zeolite. ... For the cities, see Basalt, Colorado and Basalt, Idaho. ... Komatiites are ultramafic mantle-derived volcanic rocks. ... For other uses, see granite (disambiguation). ... Categories: Mineral stubs | Igneous rocks ... Gabbro specimen. ... Peridotite xenolith from San Carlos, southwestern United States. ... It has been suggested that this article or section be merged with Dolerite. ... This page is about a volcanic rock. ... A sample of andesite (dark groundmass) with amygdaloidal vesicules filled with zeolite. ... For the cities, see Basalt, Colorado and Basalt, Idaho. ... Komatiites are ultramafic mantle-derived volcanic rocks. ... A QAPF diagram is a double triangle diagram which is used to classify igneous rocks based on mineralogic composition. ...


Example of classification

Granite is an igneous intrusive rock (crystallized at depth), with felsic composition (rich in silica and predominately quartz plus potassium-rich feldspar plus sodium-rich plagioclase) and phaneritic, subeuhedral texture (minerals are visible to the unaided eye and commonly some of them retain original crystallographic shapes). For other uses, see granite (disambiguation). ... For other uses, see Quartz (disambiguation). ... This article does not cite any references or sources. ... Lunar Ferroan Anorthosite #60025 (Plagioclase Feldspar). ... Euhedral refers to well formed crystals with sharp easily recognised faces. ...


Magma origination

The Earth's crust averages about 35 kilometers thick under the continents, but averages only some 7-10 kilometers beneath the oceans. The continental crust is composed primarily of sedimentary rocks resting on crystalline basement formed of a great variety of metamorphic and igneous rocks including granulite and granite. Oceanic crust is composed primarily of basalt and gabbro. Both continental and oceanic crust rest on peridotite of the mantle. The thickness of the Earths crust (km). ... Age of oceanic crust Oceanic crust is the part of Earths lithosphere that surfaces in the ocean basins. ... Modern petrology defnes a granulite sensuo stricto as a coarse grained, high-grade metamorphic rock composed primarily of pyroxene, plagioclase feldspar and accessory garnet, oxide and amphibole. ... For other uses, see granite (disambiguation). ... For the cities, see Basalt, Colorado and Basalt, Idaho. ... Gabbro specimen. ... Peridotite xenolith from San Carlos, southwestern United States. ...


Rocks may melt in response to a decrease in pressure, to a change in composition such as an addition of water, to an increase in temperature, or to a combination of these processes.


Other mechanisms, such as melting from impact of a meteorite, are less important today, but impacts during accretion of the Earth led to extensive melting, and the outer several hundred kilometers of our early Earth probably was an ocean of magma. Impacts of large meteorites in last few hundred million years have been proposed as one mechanism responsible for the extensive basalt magmatism of several large igneous provinces. Oceanic-continental convergence: The required conditions for plate accretion Accretion, in geology, is a process by which sediment is added to a tectonic plate. ... For the cities, see Basalt, Colorado and Basalt, Idaho. ... Large Igneous provinces (LIPS) were originally defined by Coffin and Eldholm (1992) as areas of Earths surface that contain very large volumes of magmatic rocks (typically basalt but including rhyolites) erupted over extremely short geological time intervals of a few million years or less. ...


Decompression

Decompression melting which occurs because of a decrease in pressure. The solidus temperatures of most rocks (the temperatures below which they are completely solid) increase with increasing pressure in the absence of water. Peridotite at depth in the Earth's mantle may be hotter than its solidus temperature at some shallower level. If such rock rises during the convection of solid mantle, it will cool slightly as it expands in an adiabatic process, but the cooling is only about 0.3°C per kilometer. Experimental studies of appropriate peridotite samples document that the solidus temperatures increase by 3°C to 4°C per kilometer. If the rock rises far enough, it will begin to melt. Melt droplets can coalesce into larger volumes and be intruded upwards. This process of melting from upward movement of solid mantle is critical in the evolution of the earth. In chemistry, materials science, and physics, the solidus is a line on a phase diagram below which a given substance is stable in the solid phase. ... Earth cutaway from core to exosphere. ... Mantle convection is the slow creeping motion of Earths rocky mantle in response to perpetual gravitationally unstable variations in its density. ... In thermodynamics, an adiabatic process or an isocaloric process is a thermodynamic process in which no heat is transferred to or from the working fluid. ... Peridotite xenolith from San Carlos, southwestern United States. ...


Decompression melting creates the ocean crust at mid-ocean ridgess. Decompression melting caused by the rise of mantle plumes is responsible for creating ocean islands like the Hawaiian islands. Plume-related decompression melting also is the most common explanation for flood basalts and oceanic plateaus (two types of large igneous provinces), although other causes such as melting related to meteorite impact have been proposed for some of these huge volumes of igneous rock. An oceanic ridge is an underwater mountain range, usually formed by plate tectonics. ... A lava lamp illustrates the basic concept of a mantle plume. ... Map of the Hawaiian Islands, a chain of islands that stretches 2,400 km in a northwesterly direction from the southern tip of the Island of Hawaii. ... Moses Coulee showing multiple flood basalt flows of the Columbia River Basalt Group. ... An oceanic plateau is an undersea large igneous province, the equivalent of continental flood basalts such as the Deccan Traps in India and the Snake River Plain in the Pacific Northwest of the United States. ... Large Igneous provinces (LIPS) were originally defined by Coffin and Eldholm (1992) as areas of Earths surface that contain very large volumes of magmatic rocks (typically basalt but including rhyolites) erupted over extremely short geological time intervals of a few million years or less. ...


Effects of water and carbon dioxide

The change of rock composition most responsible for creation of magma is the addition of water. Water lowers the solidus temperature of rocks at a given pressure. For example, at a depth of about 100 kilometers, peridotite begins to melt near 800°C in the presence of excess water, but near or above about 1500°C in the absence of water.[4] Water is driven out of the oceanic lithosphere in subduction zones, and it causes melting in the overlying mantle. Hydrous magmas of basalt and andesite composition are produced directly and indirectly as results of dehydration during the subduction process. Such magmas and those derived from them build up island arcs such as those in the Pacific ring of fire. These magmas form rocks of the calc-alkaline series, an important part of continental crust. In chemistry, materials science, and physics, the solidus is a line on a phase diagram below which a given substance is stable in the solid phase. ... The tectonic plates of the lithosphere on Earth. ... Categories: Geology stubs | Plate tectonics ... For the cities, see Basalt, Colorado and Basalt, Idaho. ... A sample of andesite (dark groundmass) with amygdaloidal vesicules filled with zeolite. ... An island arc is a type of archipelago formed by plate tectonics as one oceanic tectonic plate subducts under another and produces magma. ... “The Ring of Fire” redirects here. ... Calc-alkaline, or calc-alkalic series rocks, are igneous rocks which share a trend of alkali and calcium enrichment. ... The thickness of the Earths crust (km). ...


The addition of carbon dioxide is relatively a much less important cause of magma formation than addition of water, but genesis of some silica-undersaturated magmas has been attributed to the dominance of carbon dioxide over water in their mantle source regions. In the presence of carbon dioxide, experiments document that the peridotite solidus temperature decreases by about 200°C in a narrow pressure interval at pressures corresponding to a depth of about 70 km. At greater depths, carbon dioxide can have more effect: at depths to about 200 km, the temperatures of initial melting of a carbonated peridotite composition were determined to be 450°C to 600°C lower than for the same composition with no carbon dioxide.[5] Magmas of rock types such as nephelinite, carbonatite, and kimberlite are among those that may be generated following an influx of carbon dioxide into mantle at depths greater than about 70 km. Carbon dioxide is a chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom. ... Normative mineralogy is a geochemical calculation of the whole rock geochemistry of a rock sample which estimates the idealised mineralogy of a rock according to the principles of geochemistry. ... This page is a candidate to be moved to Wiktionary. ... Carbonatites are intrusive igneous rock structures with more than 50% carbonate content, many of which contain distinctive abundances of apatite, magnetite, barite, and fluorite, that may contain economic or anomalous concentrations of rare earth elements, phosphorus, niobium, uranium, thorium, copper, iron, titanium, barium, fluorine, zirconium, and other rare or incompatible... Hewn kimberlite core sample from the James Bay Lowlands region of Northern Ontario, Canada. ...


Temperature increase

Increase of temperature is the most typical mechanism for formation of magma within continental crust. Such temperature increases can occur because of the upward intrusion of magma from the mantle. Temperatures can also exceed the solidus of a crustal rock in continental crust thickened by compression at a plate boundary. The plate boundary between the Indian and Asian continental masses provides a well-studied example, as the Tibetan Plateau just north of the boundary has crust about 80 kilometers thick, roughly twice the thickness of normal continental crust. Studies of electrical resistivity deduced from magnetotelluric data have detected a layer that appears to contain silicate melt and that stretches for at least 1000 kilometers within the middle crust along the southern margin of the Tibetan Plateau.[6] Granite and rhyolite are types of igneous rock commonly interpreted as products of melting of continental crust because of increases of temperature. Temperature increases also may contribute to the melting of lithosphere dragged down in a subduction zone. In chemistry, materials science, and physics, the solidus is a line on a phase diagram below which a given substance is stable in the solid phase. ... Bridge across the Álfagjá rift valley in southwest Iceland, the boundary of the Eurasian and North American continental tectonic plates. ... Tibet Autonomous Region, Qinghai Province and Sichuan Province of China lie on the Tibetan Plateau. ... Electrical resistivity (also known as specific electrical resistance) is a measure of how strongly a material opposes the flow of electric current. ... Magnetotellurics (MT) is a natural-source, electromagnetic geophysical method of imaging structures below the earths surface. ... In chemistry, a silicate is a compound containing an anion in which one or more central silicon atoms are surrounded by electronegative ligands. ... For other uses, see granite (disambiguation). ... This page is about a volcanic rock. ... The tectonic plates of the lithosphere on Earth. ... Categories: Geology stubs | Plate tectonics ...


Magma evolution

Most magmas are only entirely melt for small parts of their histories. More typically, they are mixes of melt and crystals, and sometimes also of gas bubbles. Melt, crystals, and bubbles usually have different densities, and so they can separate as magmas evolve. Igneous differentiation is an umbrella term for the various processes by which magmas undergo bulk chemical change during the partial melting process, cooling, emplacement of eruption. ... This article is about the type of rock. ...


As magma cools, minerals typically crystallize from the melt at different temperatures (fractional crystallization). As minerals crystallize, the composition of the residual melt typically changes. If crystals separate from melt, then the residual melt will differ in composition from the parent magma. For instance, a magma of gabbroic composition can produce a residual melt of granitic composition if early formed crystals are separated from the magma. Gabbro may have a liquidus temperature near 1200°C, and derivative granite-composition melt may have a liquidus temperature as low as about 700°C. Incompatible elements are concentrated in the last residues of magma during fractional crystallization and in the first melts produced during partial melting: either process can form the magma that crystallizes to pegmatite, a rock type commonly enriched in incompatible elements. Bowen's reaction series is important for understanding the idealised sequence of fractional crystallisation of a magma. For other uses, see Mineral (disambiguation). ... Crystal (disambiguation) Insulin crystals A crystal is a solid in which the constituent atoms, molecules, or ions are packed in a regularly ordered, repeating pattern extending in all three spatial dimensions. ... In chemistry, Fractional Crystallization is a method of refining substances based on differences in soluability. ... Gabbro specimen. ... For other uses, see granite (disambiguation). ... Gabbro specimen. ... In chemistry, materials science, and physics, the liquidus is a line on a phase diagram above which a given substance is stable in the liquid phase. ... For other uses, see granite (disambiguation). ... Incompatible element is a term used in petrology and geochemistry. ... This article or section does not cite its references or sources. ... Within the field of geology, Bowens reaction series is the work of the petrologist, Norman L. Bowen who was able to explain why certain types of minerals tend to be found together while others are almost never associated with one another. ...


Magma composition can be determined by processes other than partial melting and fractional crystallization. For instance, magmas commonly interact with rocks they intrude, both by melting those rocks and by reacting with them. Magmas of different compositions can mix with one another. In rare cases, melts can separate into two immiscible melts of contrasting compositions. In chemistry, Fractional Crystallization is a method of refining substances based on differences in soluability. ...


There are relatively few minerals that are important in the formation of common igneous rocks, because the magma from which the minerals crystallize is rich in only certain elements: silicon, oxygen, aluminium, sodium, potassium, calcium, iron, and magnesium. These are the elements which combine to form the silicate minerals, which account for over ninety percent of all igneous rocks. The chemistry of igneous rocks is expressed differently for major and minor elements and for trace elements. Contents of major and minor elements are conventionally expressed as weight percent oxides (e.g., 51% SiO2, and 1.50% TiO2). Abundances of trace elements are conventionally expressed as parts per million by weight (e.g., 420 ppm Ni, and 5.1 ppm Sm). The term "trace element" typically is used for elements present in most rocks at abundances less than 100 ppm or so, but some trace elements may be present in some rocks at abundances exceeding 1000 ppm. The diversity of rock compositions has been defined by a huge mass of analytical data -- over 230,000 rock analyses can be accessed on the web through a site sponsored by the U. S. National Science Foundation (see the External Link to EarthChem). Not to be confused with Silicone. ... This article is about the chemical element and its most stable form, or dioxygen. ... Aluminum redirects here. ... For sodium in the diet, see Salt. ... General Name, symbol, number potassium, K, 19 Chemical series alkali metals Group, period, block 1, 4, s Appearance silvery white Standard atomic weight 39. ... For other uses, see Calcium (disambiguation). ... General Name, symbol, number iron, Fe, 26 Chemical series transition metals Group, period, block 8, 4, d Appearance lustrous metallic with a grayish tinge Standard atomic weight 55. ... General Name, symbol, number magnesium, Mg, 12 Chemical series alkaline earth metals Group, period, block 2, 3, s Appearance silvery white solid at room temp Standard atomic weight 24. ... The silicate minerals make up the largest and most important class of rock-forming minerals. ...


Etymology

The word "igneous" is derived from the Latin igneus, meaning "of fire". Volcanic rocks are named after Vulcan, the Roman name for the god of fire.
Intrusive rocks are also called plutonic rocks, named after Pluto, the Roman god of the underworld. For other uses, see Latin (disambiguation). ... The Forge of Vulcan by Diego Velasquez, (1630). ... Ancient Rome was a civilization that grew from a small agricultural community founded on the Italian Peninsula circa the 9th century BC to a massive empire straddling the Mediterranean Sea. ... Pluto is an alternate name for the Greek god Hades, but was more often used in Roman mythology in their presentation of the god of the underworld. ...


See also

Gem animals. ... This page is intended as a list of all rock types. ... Large Igneous provinces (LIPS) were originally defined by Coffin and Eldholm (1992) as areas of Earths surface that contain very large volumes of magmatic rocks (typically basalt but including rhyolites) erupted over extremely short geological time intervals of a few million years or less. ...

Footnotes

  1. ^ Fisher, R. V. & Schmincke H.-U., (1984) Pyroclastic Rocks, Berlin, Springer-Verlag
  2. ^ Cross, W. et al. (1903) Quantitative Classification of Igneous Rocks, Chicago, University of Chicago Press
  3. ^ This article incorporates text from the Encyclopædia Britannica Eleventh Edition article "Petrology", a publication now in the public domain.[1]
  4. ^ T. L. Grove, N. Chatterjee, S. W. Parman, and E. Medard, (2006)The influence of H2O on mantle wedge melting. Earth and Planetary Science Letters, v. 249, p. 74-89
  5. ^ R. Dasgupta and M. M. Hirschmann (2007) Effect of variable carbonate concentration on the solidus of mantle peridotite. American Mineralogist, v. 92, p. 370-379
  6. ^ M. J. Unsworth et al. (2005) Crustal rheology of the Himalaya and Southern Tibet inferred from magnetotelluric data. Nature, v. 438, p. 78-81

Encyclopædia Britannica, the eleventh edition The Encyclopædia Britannica Eleventh Edition (1910–1911) is perhaps the most famous edition of the Encyclopædia Britannica. ... The public domain comprises the body of all creative works and other knowledge—writing, artwork, music, science, inventions, and others—in which no person or organization has any proprietary interest. ...

References

  • R. W. Le Maitre (editor) (2002) Igneous Rocks: A Classification and Glossary of Terms, Recommendations of the International Union of Geological Sciences, Subcommission of the Systematics of Igneous Rocks., Cambridge, Cambridge University Press ISBN 0-521-66215-X

External links

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  Results from FactBites:
 
Igneous Rock Identification Exercise (1238 words)
Igneous rocks are crystalline or glassy rocks formed by the cooling and solidification of molten magma.
Rocks formed from the cooling and solidification of magma deep within the crust are distinct from those erupted at the surface mainly owing to the differences in conditions in the two environments.
As discussed earlier, texture is used to subdivide igneous rocks into two major groups: (1) the plutonic rocks, with mineral grain sizes that are visible to the naked eye, and (2) the volcanic types, which are usually too fine-grained or glassy for their mineral composition to be observed without the use of a microscope.
  More results at FactBites »

 
 

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